One idea that seems to work is to purchase inexpensive sweat pants or light weight pull-on pants (try Goodwill or similar stores). Leave the healthy leg of the pants intact. Cut open the pants leg on the side where the cast is, on the outer part of the pants leg. Cut open as high up as the top of the cast. Then apply Velcro patches in a couple of places - you can sew them on, or get iron-on Velcro strips. You don't need too many, just one near the hip or top of the cast so that no underwear is visible, and maybe one near the knee and one at the calf or ankel to keep the pants leg from flopping around. When you help your daughter get dressed, she just slips her healthy leg into the pants, and you snap the Velcro patches or strips together on the casted leg.
As for a bath, I wouldn't risk it. Too many possibilities of slipping, trying to balance, getting the cast wet, etc. Since this will last over a month, use a hand held shower head to wash her hair (she can sit next to the tub, bend her head back a little, and you just wet her hair, lather it, rinse). Or take her to a hair salon once a week and have her hair shampooed by a stylist in those handy sinks they have. For daily cleansing, get baby wipes. You can also purchase large wipes intended for bed-bound patients, or for anyone unable to access a bath tub for whatever reason, at larger places like Walgreens or CVS, the kind of larger pharmacy that sells or rents wheelchairs, crutches and canes, etc, or at medical supply stores. Or go to Amazon and type in adult wipes in the search window, and you'll see many. They're larger than baby wipes, stronger, and intended for this exact purpose.
I'd also keep her out of school. Ask the teacher for some ideas on how to keep her involved (with some simple work to do at home, similar to what she'd be doing in school).
Most of all, don't stress too much about her missing school, or missing a bath. She's young - let her have this time to relax and heal. Healing takes a lot of calories and energy - she'll probably be tired, and most likely the cast will itch or be hot or otherwise uncomfortable. This is not the time to worry about too many cartoons, or playing on the iPad. Make sure she knows that when she's back in school, after the cast comes off, she'll be back to her usual routine and restrictions about watching tv, etc, but while she's healing, let her have a little fun. Get her some Play-Doh, or new colored pencils, or Legos or something to keep her rested but that also gives her something to do.